FIG. 1 shows a block diagram of an optical disk apparatus 1.
The optical disk apparatus 1 includes a spindle motor 11, an optical device 12, a control unit 13, and a signal-processing unit 14. The optical. disk apparatus 1 is provided with an optical disk 2. The optical disk 2 is engaged by the spindle motor 11. The spindle motor 11 rotates the optical disk 2 at a predetermined rotational speed according to directions from the control unit 13.
The optical device 12 irradiates light to the optical disk 2. The light is reflected by the optical disk 2, and the light reflected by the optical disk 2 is supplied to the optical device 12. The optical device 12 detects the reflected light from the optical disk 2, and outputs a signal that is detected. The detected signal output from the optical device 12 is supplied to the signal-processing unit 14. The signal-processing unit 14 demodulates and decodes the detected signal, and information recorded on the optical disk 2 is obtained. The information decoded by the signal-processing unit 14 is supplied to an external storage apparatus 3.
Various optical recording media are currently available, such as compact disks (CD), and digital video disks (DVD). The optical disk apparatus 1 is required to be compatible with the various optical recording media.
A high data density is obtained if a semiconductor laser included in the optical device 12 irradiates short wavelength light. The diameter of an optical spot formed by a condensing optical system with a given numerical aperture (Numerical Aperture, NA) is proportional to the wavelength of the light to be used. By using a short wavelength laser for reading and writing information, recording pits can be made small, and high density can be attained. Previously, it was difficult for a semiconductor laser to generate short-wavelength light, because the gain required for laser oscillation was difficult to obtain. However, recently and continuing, semiconductor lasers capable of oscillating at a wavelength of 410 nm at normal temperature for a long time are being commercialized, and are used in optical disk apparatuses. Further, research on recording materials for short-wavelength is advancing.
The condensing spot can be made small, when the wavelength is held constant, by increasing NA of the condensing optical system. For example, NA of a pickup for a CD is 0.45, and NA of an objective lens for a DVD is 0.60.
In order to avoid collisions between the disk and the lens, the optical disk apparatus 1 is provided with a sufficient working distance. Further, weight of the objective lens provided to a carriage must be as light as possible in order to facilitate movement. For this reason, the objective lens cannot be thick.
It is possible to realize a thin lens having a high NA by designing the lens surface as an aspheric surface that is defined by a high order polynomial. However, in the past it was difficult to manufacture a lens with the required precision. Through improvements in processing technology, such an objective lens that can be applied to an optical disk apparatus has at last become available.
Further, requirements concerning aberration of the lens having a high NA are also severe, coma aberration generated by inclination of a medium increasing in proportion to the third power of the NA. The influence of the aberration is reduced by making substrate thickness of the disk less than conventional optical disks. For example, a CD having a diameter of 120 mm, and having a capacity of 640 MB, is 1.2 mm thick, while a DVD that has the same diameter, and a capacity of 4.7 GB uses two substrates, each of which is 0.6 mm thick, that is, 1.2 mm thick in total.
As mentioned above, specifications of recording media change as higher densities become available. For this reason, optical disk apparatuses are required to be capable of reading/writing not only new higher-density media but also conventional media. Accordingly, an optical head that is capable of providing sufficient optical properties to recording media of differing operating wavelength, NA and substrate thickness is needed. Considering apparatus size and manufacturing cost, it is not realistic to install separate light sources and optical systems corresponding to various media. A common configuration capable of reading/writing different media is required.
Conventionally, a method is considered, whereby the objective lens is common. However, it is difficult to eliminate the aberration generated by the difference in substrate thickness. Especially, in the case of a separated (two body) optical system that is designed for high-speed access, wherein the light source and the detection system are fixed, and only the objective lens moves for seeking; since the objective lens moves extensively in relation to the light source, the light incident on the objective lens cannot be greatly different from parallel light. If the incident light turns into divergent or convergent light, the luminous intensity of light changes according to whether the objective lens is near the center of a disk or near the edge of it, and performance is degraded. Accordingly, control of the aberration of the incident light is difficult.
Conversely, when the light source common, the configuration is such that a short wavelength light source is used, and the light is made to pass along an optical path that is different depending on the kind of medium, and different objective lenses are used. Since each objective lens is designed for a substrate for reading/writing at the optimal wavelength to be used, even if there is a difference in substrate thickness, it is easy to suppress the aberration.
Further, NA is determined such that the required spot is obtained, considering the difference in optimal wavelength. About optimization of the diameter of the spot, the difference in optimal wavelength can be compensated for by setup of the NA of an optical system.
The wavelength dependability of a medium and a method of optical-path switching pose problems. The wavelength dependability of the medium appears as a reduction of the signal properties when the wavelength shifts from the optimal wavelength. The problem of wavelength dependability can be solved by designing the optical system so that the resolution is high, and a wide margin of tolerance is provided for normal reading/writing operations of the signal.
On the other hand, as for the problem related to switching of the optical path, a method wherein two objective lenses are mounted to a switching mechanism that switches by rotation is used.
A block diagram of an example of the conventional optical system is shown in FIG. 2.
The conventional optical system shown in FIG. 2 adopts a one-body optical head with all components installed on a carriage.
A one-body optical pickup 20 shown in FIG. 2 includes an integrated optical head 21, a collimating lens 22, a mirror 23, objective lenses 24 and 25, and a stage 26, all of which are mounted on a carriage 27.
The integrated optical head 21 is an optical device that further includes a light source, a detector for focal error detection, a detector for tracking error detection, and a detector for reproducing-signal detection, all of which are integrated. The light that is irradiated from the integrated optical head 21 is incident to the collimating lens 22. The collimate lens 22 changes the divergence light from the integrated optical head 21 into parallel light. The light output by the collimating lens 22 is incident on the mirror 23. The mirror 23 reflects the light from the collimating lens 22 in the direction of the disk 2, i.e., the direction of arrow B.
The light reflected by the mirror 23 is converged by one of the objective lens 24 or the objective lens 25, and is irradiated to the disk 2. The light irradiated to the disk 2 is reflected by the disk 2, and passes through the objective lens 24 or 15, the mirror 23, and the collimating lens 22 again, and is supplied to the integrated optical head 21.
The objective lenses 24 and 25 are fixed to the stage 26. The stage 26 is arranged so that it can rotate in the direction of arrow C. When the stage 26 rotates, either of the objective lenses 24 or 25 is located above the mirror 23, i.e., the objective lens is switched. The switching of the optical system is carried out in this manner.
The rotating mechanism of the stage 26 (not shown) has to be large in size in order to attain precision. If the stage 26 is enlarged, the mass of the optical pickup 20 becomes large. When the mass of the optical pickup 20 becomes large, there are problems, such as track-seeking speed being decreased.
Further, the optical pickup 20 shown in FIG. 2 requires adjustments of the objective lenses 24 and 25, and adjustments of the switching mechanism, i.e., axial adjustment of the stage 26, which poses problems, such as the assembly process becoming complicated.
For this reason, it is desired that an optical-path switching mechanism of a separated optical system that is capable of high-speed seeking be provided to the fixed optical unit.
The present invention is made in view of the above-mentioned desire, and aims at offering an optical device, and an information recording/reproducing apparatus using the optical device, wherein optical-path switching is made possible using a separated optical system.
Further, the present invention aims at offering an optical device, and an information recording/reproducing apparatus using the optical device, that can provide an optical path with high precision using a separated optical system.